1. Field of the Invention
The present invention relates to audio amplifier circuits, and more particularly relates to an amplifier circuit and method for improving the efficiency, performance and power of amplifier circuits for driving loudspeakers.
2. Description of the Prior Art
Electromechanical transducers, such as loudspeakers, can be considered equivalent to linear electric motors. By analogy to such an equivalent electric motor, electrical energy (watts) passes through the voice coil of a loudspeaker, causing the coil, and anything attached to it, to move. As the coil moves through the magnetic field an electrical voltage is generated in the coil. In order for the exact values of electrical power flowing through the coil to be calculated this generated voltage must be considered.
FIG. 1 illustrates the equivalent electrical properties of a typical prior art loudspeaker circuit 10. In the circuit inductor 12 represents the electrical inductance of the coil. Resistor 14 represents the DC resistance of the coil, and 16 represents the voltage generated by the motion of the coil in the magnetic field.
FIG. 2 shows a schematic equivalent circuit 18 which illustrates the effect the generated voltage has on the electrical power in a loudspeaker coil. A pair of voltage dividing resistors 20 and 22 as well as a current sampling resistor 24 are added to the inductor 12' and resistor 14' of the equivalent circuit of FIG. 1. A state of the art audio power amplifier 26 is connected to the loudspeaker through line 28 with the other side of the speaker connected to the amplifier ground through line 29 and resistor 24.
A suitable dual trace oscilloscope, not shown, is connected to points E and I of circuit 18, with the oscilloscope ground connected to line 29. A positive voltage pulse E with duration time T.sub.n is applied to amplifier input 30. The gain is adjusted until approximately 6 volts appear at the output line 28.
FIG. 3-A shows the oscilloscope traces which represent the voltage applied to the speaker, while FIG. 3-B shows the traces for the current I flowing through the voice coil.
At some time (T.sub..alpha.) the amplifier 26 delivers a voltage of 6 volts to the loudspeaker. At first, due to the delay caused by the electrical inductance 12', no current flows through the coil and therefore no power is flowing through the coil. At a second time T1, enough current is flowing to produce power in the coil. The coil starts to accelerate outward. It reaches a maximum velocity at T2. At T3 the energy flowing through the coil is equal to the force of the suspension. At T3 the coil comes to rest.
The current I, and hence the power in the system, contains a component which is proportional to the voltage generated by the motion of the coil. It is this component which represents an error in the electrical power input to the coil. In a typical loudspeaker enclosed in a ported enclosure, the error component produced by voltage generated by the motion of the coil can equal 50% of the applied voltage.